The present invention relates to a process and an apparatus for the manufacture of gases containing hydrogen and carbon monoxide, i.e., synthesis gas, by the partial combustion of carbonaceous fuels, and for cooling of the hot gases thus obtained.
Efficient cooling of hot gases obtained in the partial combustion of carbonaceous fuels in a waste heat boiler generating high-pressure steam is difficult to achieve for a number of reasons.
It is not desirable to cool the gases in a conventional straight tube waste heat boiler in which the gases pass through straight tubes which are externally cooled with water, because the gases contain soot particles which deposit on the walls of the tubes and eventually block them. It is therefore preferable to cool the gases in a helical tube waste heat boiler in which the gases pass through helical tubes which are externally cooled by water, since soot build-up does not occur in the helical tubes. Such helical tube waste heat boilers are described, for example, in U.S. Pat. No. 2,967,515 to Hofstede et al and Canadian Pat. No. 634,687 to Hofstede et al.
In order to operate a helical tube waste heat boiler as efficiently as possible, it is desirable to generate very high pressure steam. Accordingly, a large pressure differential results between the water vapor outside the helical tubes and the gases inside the helical tubes. In addition, the temperature of the helical tubes is very hot as a consequence of contact with the extremely hot combustion gases. Under such severe conditions the helical tubes are liable to collapse because of weaknesses in the tubes caused by unroundness.
Unroundness of a tube signifies its deviation from perfect radial and axial symmetry. Such unroundness results in a helical tube during its manufacture by the winding of a straight tube and considerably reduces its mechanical strength. The possibility of tube collapse therefore limits the extent to which the pressure differential between the inside and outside of the helical tubes can be raised at high tube temperatures. Because of these limitations the advantages of very high steam generation cannot be realized in helical tube waste heat boilers unless tubes having a low degree of unroundness are employed which are difficult to produce and expensive, or unless special measures are taken to reduce the temperature and/or pressure in the boiler such as described in U.S. Pat. No. 3,712,371 to Ter Haar et al. This patent discloses a process for cooling a hot synthesis gas in a waste heat boiler containing a straight tube of critical length attached to a helical coil tube, both of which are in external contact with water within the waste heat boiler. The present invention concerns a process and apparatus wherein a conventional helical tube waste heat boiler is employed in conjunction with an external primary cooling zone of unique design as hereinafter described.
It is therefore an object of the present invention to provide a process for the manufacture of gases containing hydrogen and carbon monoxide in which cooling of the hot gases is carried out in a helical tube waste heat boiler operating at high pressure differentials in an economical, efficient and reliable manner. It is another object of the present invention to cool hot hydrogen and carbon monoxide containing gases in large scale helical tube waste heat boilers which would be prohibitively expensive to construct if it were necessary to employ large diameter helical tubes having a low degree of unroundness.